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EP-2017-195

(Submitted on 21 Aug 2017)

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Abstract

Measurements of $CP$ observables in $B^\pm \rightarrow D^{(*)} K^\pm$ and
$B^\pm \rightarrow D^{(*)} \pi^\pm$ decays are presented, where $D^{(*)}$
indicates a neutral $D$ or $D^*$ meson that is an admixture of $D^{(*)0}$ and
$\bar{D}^{(*)0}$ states. Decays of the $D^*$ meson to the $D\pi^0$ and
$D\gamma$ final states are partially reconstructed without inclusion of the
neutral pion or photon, resulting in distinctive shapes in the $B$ candidate
invariant mass distribution. Decays of the $D$ meson are fully reconstructed in
the $K^\pm \pi^\mp$, $K^+ K^-$ and $\pi^+ \pi^-$ final states. The analysis
uses a sample of charged $B$ mesons produced in $pp$ collisions collected by
the LHCb experiment, corresponding to an integrated luminosity of 2.0, 1.0 and
2.0 fb$^{-1}$ taken at centre-of-mass energies of $\sqrt{s}$ = 7, 8 and 13 TeV,
respectively. The study of $B^{\pm} \to D^{*} K^{\pm}$ and $B^{\pm} \to D^{*}
\pi^{\pm}$ decays using a partial reconstruction method is the first of its
kind, while the measurement of $B^{\pm} \to D K^{\pm}$ and $B^{\pm} \to D
\pi^{\pm}$ decays is an update of previous LHCb measurements. The $B^{\pm} \to
D K^{\pm}$ results are the most precise to date.

Figures and captions

Invariant mass distributions of selected $ B ^\pm \rightarrow [ K ^\pm \pi ^\mp ]_{D}h^{\pm}$ candidates, separated
by charge, with $ B ^- {\rm(} B ^+ {\rm)}$ candidates on the left (right).
The top panels contain the $ B ^\pm \rightarrow D^{(*)0}K^{\pm}$ candidate samples, as defined by a PID requirement on the companion particle.
The remaining candidates are placed in the bottom panels, reconstructed with a pion hypothesis for the companion.
The result of the fit is shown by the thin solid black line, and each component is listed in the legend. The component referred to as `Part. reco. mis-ID' is the total contribution from all partially reconstructed and misidentified decays.

Systematic uncertainties for the $ C\!P$ observables measured in a fully reconstructed manner, quoted as a percentage of the statistical uncertainty on the observable. The Sim uncertainty on $R_{K/\pi}^{K\pi}$ is due to the limited size of the simulated samples used to determine the relative efficiency for reconstructing and selecting $ B ^- \rightarrow D \pi ^- $ and $ B ^- \rightarrow D K ^- $ decays.